commutator press a pair of metallic brushes or springs, as contact pieces, which touch opposite sides at points

above and below, and so lead away into the circuit the current generated in the coils of the rotating armature. Suppose the lines-of-force in the field to run from right to left,1 and the armature to rotate left-handedly, as seen in Fig. 151, then, by the rule given in Art. 395, in all

1 Their direction is not exactly thus when the generator is working, as the magnetic force due to the currents in the coils, which is nearly horizontal in direction, changes the resultant magnetic force to an oblique direction across the field. It is for this reason that the commutator "brushes" have to be displaced with a certain angular "lead." A similar displacement of the brushes occurs in the Gramme and all other dynamo-electric generators, the degree of displacement to get maximum strength of current varying with the resistances in the external circuit and with the work done by the current.

the separate wires of the coils, moving upwards on the right, there will be currents induced in a direction from the back toward the front. In all the separate wires of the coils moving downwards on the left of the axis, the induced currents will be in a direction from the front toward the back. Hence, if the coils are joined as described to the commutator bars all the currents thus generated in one half of the coils will be flowing into the external circuit at one of the commutator brushes; and all the reverse currents of the other half of the coils will be flowing out of the other brush. The terminal screws connected by wires to the commutator brushes correspond to the + and - poles of a galvanic battery, the coils of the field-magnets being included in the external circuit.

410. Gramme's Machine. In 1864 Pacinotti invented a magneto-electric machine, its armature being a toothed ring of iron with coils wound between the projections. In 1870 Gramme invented a dynamo-electric machine having a ring armature differing only in being completely overwound with coils of insulated copper wires. The principle of this generator is shown in diagram in Fig. 152. The ring itself, made of a bundle of annealed iron wires, is wound in separate sections, the ends of each coil being joined to strips of copper which are insulated from each other, and fixed symmetrically as a commutator around the axis, like a split tube. Their actual arrangement is shown again in Fig. 153. The coils of the separate sections of the ring are connected together in series, each strip of the commutator being united to one end of each of two adjacent coils. Against the split-tube collector press metallic brushes to receive the current. When this ring is rotated the action is as follows:-Suppose (in Fig. 152) the ring to rotate in the opposite direction to the hands of a clock in the magnetic field between the N and S-poles of a magnet (or electro-magnet), and that the positive direc

tion of the lines of force is from N to S.

As a matter

of fact the lines will not be straight across from N to S, because the greater part of them will pass into the ring near N and traverse the iron of the ring to near S, where they emerge; the space within the ring being almost entirely destitute of them. Consider one single coil of the wire wrapped round the ring at E" which is ascending

toward S; the greatest number of lines-of-force will pass through its plane when it lies near E", at right angles to the line NS. As it rises toward S and comes to E the number of lines-of-force that traverse it will be steadily diminishing, and will reach zero when it comes close to S and lies in the line NS, edgeways to the lines-of-force. As it moves on toward E' it will again enclose lines-offorce, which will, however, pass in the negative direction through its plane, and at E' the number of such negative lines-of-force becomes a maximum. Hence, through all its journey from E" to E' the number of (positive) linesof-force embraced by a strand of the coils has been diminishing; during its journey round the other half from E' to E" again, the number will be increasing. Therefore, by the rule given in Art. 395, in all the coils moving round the upper half of the ring direct currents are being

generated, while in the coils of the lower half of the ring inverse currents are being generated. Hence there is a constant tendency for electricity to flow from the left side at E' both ways round towards the right side at E", and E" will be at a higher potential than E'. A continuous

current will therefore be generated in an external wire, making contact at F and F by means of brushes, for as each successive coil moves up towards the brushes the induced current in it increases in strength, because the coils on each side of this position are sending their induced currents also toward that point. Fig. 153 shows the little Gramme machine, 21 inches high, suitable for

producing an electric arc light when driven by a 21 horse-power engine. Above and below are opposite pairs of powerful electro-magnets, whose iron pole-pieces project forwards and almost embrace the central ringarmature, which, with the commutator, is fixed to the horizontal spindle.

411. (a) Brush's Machine.--In Brush's dynamoelectric generator, a ring-armature is also used, identical in form with that invented by Pacinotti, the iron ring being enlarged with protruding cheeks, with spaces between, in which the coils are wound, the coils themselves being also somewhat differently joined, each coil being united with that diametrically opposite to it, and having for the pair a commutator consisting of a collar slit into two parts. For each pair of coils there is a similar collar, the separate collars being grouped together and communicating to two or more pairs of brushes that rub against them the currents which they collect in rotating. The electromotive-force of these machines is very high, hence they are able to drive a current through a long row of arc lamps connected in one series. The largest Brush machines capable of maintaining 65 arc lights have an electromotive-force exceeding 3000 volts. Dynamo machines having modifications of the ringarmature have also been invented by Gülcher, Schuckert, and others. In Gülcher's and Schuckert's machines the ring-armature takes the form of a flattened disk. In Bürgin's machine the armature is made up of eight or ten rings, each constructed upon a very simple hexagonal core of iron wire, and placed side by side upon one spindle, each ring being set slightly in advance of its neighbour. In Crompton's dynamo the armature is wound on a hollow cylindrical core built up of flat thin iron rings.

Siemens and others have devised another class of dynamo-electric machines, differing entirely from any of the preceding, in which a coil or other movable conductor slides round one pole of a magnet and cuts the